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5.  HEREDITY  OF  HAIR-LENGTH  IN  GUINEA-PIGS 

AND  ITS  BEARING  ON  THE  THEORY 

OF  PURE  GAMETES 

BY 

W.  E.  CASTLE   AND   ALEXANDER   FORBES 


6.  THE  ORIGIN  OF  A  POLYDACTYLOUS  RACE 
OF  GUINEA-PIGS 

BY 

W.  E.  CASTLE 


PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
MAY,  1906 


CARNEGIE  INSTITUTION  OF  WASHINGTON 

PUBLICATION  No.  49 


PRESS  or 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER,  PA. 


5.  HEREDITY  OF  HAIR-LENGTH  IN  GUINEA-PIGS 

AND  ITS  BEARING  ON  THE  THEORY 

OF  PURE  GAMETES 


W.  E.  CASTLE  AND  ALEXANDER  FORBES 


PAPERS  j)F  THE  STATION  FOR  EXPERIMENTAL  EVOLUTION  AT  COLD  SPRING  HARBOR, 
NEW  YORK,  No.  5 

CONTRIBUTIONS  FROM  THE  ZOOLOGICAL  LABORATORY  OF  THE  MUSEUM  OF  COMPARATIVE 
ZOOLOGY  AT  HARVARD  COLLEGE.     E.  L.  MARK,  DIRECTOR.     No.  175 


5.    HEREDITY   OF   HAIR-LENGTH    IN   GUINEA-PIGS,   AND    ITS 
BEARING   ON   THE   THEORY   OF   PURE   GAMETES. 


BY  W.  E.  CASTLE  AND  ALEXANDER  FORBES. 


1.    INTRODUCTION. 

In  earlier  papers  (Castle,  103,  105)  it  has  been  shown  by  one  of  us  that 
long  or  "  Angora  "  coat  in  guinea-pigs  and  rabbits  is  alternative  in  heredity 
;)0to  normal  or  short  coat.  It  has  been  shown  further  that  in  cross-breeding 
normal  or  short  coat  dominates  over  long  coat,  in  conformity  with  Mendel's 
law  of  heredity,  but  that  the  purity  of  the  gametes  formed  by  cross-breds 
is  not  absolute.  Impurity  of  the  gametes  is  indicated  by  two  facts:  First, 
the  number  of  long-haired  young  produced  by  cross-bred  parents  is  in 
excess  of  the  Mendelian  proportion,  one-fourth ;  secondly,  many  of  these 
long-haired  young  show  an  imperfect  development  of  the  long-haired  char- 
acter, as  compared  with  their  long-haired  ancestors.  Both  these  facts  may 
be  explained  by  supposing  that  the  alternative  characters,  short  and  long 
'hair,  which  are  present  in  the  cross-breds — one  seen,  the  other  unseen — have 
in  many  cases  failed  to  segregate,  or  have  segregated  only  imperfectly,  when 
gametes  have  been  produced  by  the  cross-breds.  Accordingly  the  con- 
formity with  Mendel's  law  is  a  qualified  one. 

More  extended  and  detailed  studies  made  by  us  during  the  past  year 
confirm  these  conclusions  and  add  several  new  facts  concerning  the  behavior 
in  heredity  of  these  alternative  characters.  The  idea  advanced  tentatively 
by  Castle  (105),  that  the  hair-lengths  of  guinea-pigs  form  a  discontinuous 
series  of  two,  three,  four  or  more  times  the  length  of  short  or  normal  hair, 
is  found  to  be  incorrect.  Careful  examination  of  the  hair  of  several  hundred 
guinea-pigs  (mostly  cross-breds)  shows  that  there  occur  hairs  of  practically 
all  lengths  from  3.3  cm.  up  to  about  23  cm.  The  series  of  supposed  maximal 
hair-lengths  of  twice  4  cm.,  thrice  4  cm.,  etc.,  resulted  from  an  insufficient 
number  of  observations. 

5 


HEREDITY   OF   HAIR-LENGTH   IN   GUINEA-PIGS. 


2.    CHARACTER   OF   FOLLICLE   ACTIVITY   IN   THE   PRODUCTION    OF   SHORT 
AND   OF  LONG   HAIR. 

If  an  examination  is  made  of  the  longest  hairs  plucked  from  the  back 
of  an  adult  short-haired  guinea-pig,  it  is  found  that  the  hairs  narrow  toward 
the  base,  owing  to  a  diminution  in  the  diameter  of  the  medulla,  which  is 
entirely  wanting  in  the  follicle  (see  Fig.  i,  A).  Such  a  hair  has  ceased  to 
grow,  having  completed  a  definite  growth  cycle, 
and  will  sooner  or  later  be  shed.  This  growth 
cycle  was  nearing  its  end  when  the  follicle  ceased 
to  form  medullary  substance.  In  ordinary 
guinea-pigs  the  hair  stops  growing  when  it  has 
reached  a  length  of  about  4  cm.  In  long-haired 
guinea-pigs  the  hair-follicle  does  not  cease  form- 
ing medullary  substance  when  a  definite  hair- 
length  has  been  attained.  The  growth  of  the 
hair  is  indefinite,  ending  only  with  the  degenera- 
tion of  the  follicle  itself.  The  time  when  this 
occurs  is  determined  to  some  extent  by  the  physi- 
cal condition  of  the  animal.  The  lifetime  of  a 
hair-follicle  of  this  sort  may  be  prolonged  by 
good  care,  as  fanciers  well  know.  Pregnant  or 
nursing  mother  guinea-pigs  frequently  lose  their 
longest  hair.  Insufficient  or  improper  food  is 
likely  to  have  a  similar  result  in  either  sex.  Ac- 
cordingly, the  fancier  gives  careful  attention  to 
the  diet  of  long-haired  animals  intended  for  ex- 
hibition, and  often  protects  the  hair  in  special 
ways  from  mechanical  injury.  There  is,  how- 
ever, no  reason  to  suppose  that  such  care  induces 
indefinite  activity  of  the  hair-follicle.  On  the 
contrary,  animals  of  a  short-haired  race,  under 
the  best  of  care,  will  form  only  hairs  of  deter- 
minate length,  whereas  animals  of  a  long-haired 
race,  however  much  abused,  will,  so  long  as  they 
live,  continue  to  form  hairs  of  indeterminate 
growth.  The  two  types  of  hair-growth  are  quite  distinct,  and  are  sharply 
alternative  in  heredity.  They  are  probably  paralleled  in  our  own  head-  and 
body-hairs  respectively,  the  former  being  of  indeterminate,  the  latter  of 
determinate  growth. 


FIG.  i.— A,  Base    of  hair,  fully 
grown,  of  determinate  growth  type. 

B ,  Base  of  hair  of  indeterminate 
growth.   From  camera  drawings,  same 
magnification. 

C,  Cortex;  F,  follicle;  M,  me- 
dulla. 


HEREDITY   OF  HAIR-LENGTH   IN   GUINEA-PIGS.  7 

Like  ourselves,  long-haired  guinea-pigs  bear  hairs  of  both  sorts,  whereas 
short-haired  guinea-pigs,  like  our  probable  simian  ancestors,  bear  only  hairs 
of  determinate  growth.  Similar  alternative  conditions  of  hair-growth  occur 
in  many  different  kinds  of  mammals,  as  for  example  in  rabbits,  goats,  cats 
and  horses,  the  indefinite  growth  in  the  latter  case  occurring  in  the  hairs 
of  the  mane  and  tail  only  (see  Davenport,  104).  The  long-haired  (or  indefi- 
nite growth)  condition  is  doubtless  coenogenetic  in  all  cases,  and  has  probably 
arisen  independently  in  each  case  as  a  discontinuous  variation  or  mutation. 
By  selection  the  long-haired  condition  is  easily  made  a  racial  character, 
for  long-haired  animals  produce  only  long-haired  offspring.  But  without 
selection,  what  would  be  the  fate  of  this  mutation?  This  our  breeding 
experiments  may  perhaps  indicate. 

Let  us  consider  first  the  extreme  conditions  of  follicle  activity  which 
we  have  encountered  in  the  guinea-pigs  studied.  In  long-haired  guinea-pigs 
the  hairs  all  over  the  body  attain  a  greater  length  than  in  short-haired  ones, 
but  only  hairs  of  the  back  and  sides  grow  indefinitely.  Accordingly,  in  com- 
parative studies  of  the  hair-length,  it  has  been  found  convenient  to  select  a 
few  of  the  longest  hairs  to  be  found  on  the  back  or  rump  of  the  animal  and 
to  use  these  as  a  standard  of  comparison.  Following  this  method  the  hair 
of  guinea-pigs  of  different  races  has  been  measured  at  frequent  intervals 
from  birth  to  an  age  at  which  the  hair  had  attained  full  growth. 

A  litter  of  four  guinea-pigs  of  a  pure  short-haired  race  yielded  measure- 
ments which  combined  are  expressed  graphically  in  Fig.  2,  A.  Starting  at 
birth  with  a  length  of  about  18  mm.,  the  hairs  grew  very  rapidly  during  the 
first  week,  at  the  end  of  which  they  measured  about  25  mm.,  an  average 
increase  of  a  millimeter  a  day ;  in  the  second  and  third  weeks  they  grew  less 
rapidly,  measuring  about  29  mm.  at  the  end  of  two  weeks,  and  33  mm.  at  the 
end  of  three  weeks.  At  this  time  the  growth  of  the  hairs  which  the  animals 
bore  at  birth  was  practically  complete ;  the  hair- follicles  now  ceased  to  form 
medullary  substance,  and  consequently  the  hair  narrowed  to  a  base  of  solid 
cortex  (Fig.  i).  From  this  time  on  the  measurements  show  from  week 
to  week  only  slight  deviations  from  a  length  of  33  to  35  mm. 

Several  series  of  measurements  of  the  hair  of  long-haired  guinea-pigs, 
when  combined  and  expressed  graphically,  are  shown  in  D,  Fig.  2.  The 
hair-length  at  birth  is  about  the  same  as  in  animals  of  class  A,  or  even  a  little 
shorter,  but  the  growth  rate  does  not  show  the  series  of  rapid  changes  seen 
in  class  A.  Growth  progresses  very  steadily  at  an  average  rate  of  about  0.83 
mm.  a  day  during  the  first  one  hundred  days  and  at  about  0.75  mm.  a  day 
during  the  second  one  hundred  days.  But  a  glance  at  Fig.  2,  D,  shows 
that  the  slowing  up  of  the  growth  rate  is  a  very  gradual  process. 

After  the  hairs  have  attained  a  length  of  about  170  mm.  they  begin  to  fall 
out,  but  without  previously  narrowing  to  a  base,  as  in  class  A.  Rarely  does  a 


8 


HEREDITY   OF   HAIR-LENGTH    IN   GUINEA-PIGS. 


hair  attain  a  length  of  200  mm.  As  the  longest  hairs  are  shed,  new  ones 
are  found  to  be  growing  up  to  take  their  places,  but  there  is  usually  a  period, 
after  shedding  sets  in  and  before  the  second-growth  hairs  are  fully  devel- 
oped, when  the  maximum  coat  length  shows  a  decided  falling  off.  This 
is  indicated  in  D,  Fig.  2,  as  occurring  at  about  the  age  of  two  hundred  and 
and  fifty  days.  The  type  of  hair  growth  found  in  class  D  may  be  called 
indeterminate  or  continuous;  that  of  class  A,  determinate. 


Kge  in  days 


Age  da/S 


FIG.  2. 


Our  original  stock  of  short-haired  guinea-pigs  showed  considerable 
variation  in  maximal  hair-lengths,  ranging  from  35  to  50  mm.,  but  all  of 
the  determinate  type  of  growth  (classes  A  and  B,  Fig.  2).  The  great 
majority  of  them  would  probably  have  fallen  in  class  B,  but  we  can  not  be 
sure  of  this,  as  many  of  the  animals  were  no  longer  available  for  record 
when  we  began  making  hair-measurements  systematically.  The  hair-lengths 
recorded  for  class  A  are  certainly  exceptionally  short,  even  for  animals  of 
determinate  hair  growth. 


HEREDITY   OF  HAIR-LENGTH   IN   GUINEA-PIGS. 


3.     THE   EFFECTS    OF    CROSS-BREEDING    BETWEEN    SHORT-HAIRED    AND 
LONG-HAIRED    ANIMALS. 

(a)  MATING  B  X  D. 

When  the  short-haired  animals  were  mated  with  animals  of  class  D, 
only  young  of  the  former  sort  were  obtained,  which  result  shows  clearly 
the  dominant  character  of  short  hair.  However,  the  maximal  hair-length 
of  the  cross-breds  was  usually  40  to  50  mm.  (class  B,  Fig.  2)  ;  that  is,  it  fell 
within  the  upper  half  of  the  range  of  variation  of  the  pure  short-haired 
stock,  though  in  no  case,  we  think,  did  it  transcend  the  range  of  variation 
of  the  short-haired  stock  itself.  We  are  warranted,  then,  in  concluding 
that  the  dominance  in  the  offspring  of  the  short-haired  type  of  growth  is 
complete.  Type  B  mated  with  type  D  gives  only  type  B. 

(&)    MATING  B(D)XB(D). 

But  when  cross-breds  between  the  types  B  and  D  were  bred  together, 
not  only  did  type  D  reappear  after  skipping  a  generation,  but  a  new  and 
intermediate  type  was  found  also,  which  we  may  call  type  C.  This  is  not 
sharply  separated  from  types  B  and  D,  but  is  made  up  of  individuals  scat- 
tered all  the  way  between  those  parental  types.  It  contained,  first,  indi- 
viduals whose  hair  grew  continuously  from  the  age  of  twenty  days  on,  but 
much  more  slowly  than  does  the  hair  of  class  D,  with  a  tendency  to  break 
off  at  lengths  much  less  than  those  attained  in  class  D.  The  long  hairs  of 
such  animals  were  also  frequently  less  numerous  than  in  typical  class  D, 
as  if  part  of  the  hairs  only  were  continuous  in  growth,  while  the  others 
were  determinate.  In  these  individuals  we  see  the  alternative  characters 
coexisting  as  in  a  mosaic.  In  other  cases  animals  placed  in  class  C  appeared 
to  have  only  hair  of  determinate  growth,  but  growth  continued  until  a 
length  of  60  to  80  mm.  had  been  attained  before  the  hair  narrowed  to  a  base. 
Such  animals  represent  an  intimate  blend  rather  than  a  mosaic  of  the  con- 
trasted types  B  and  D,  yet  with  a  closer  approximation  to  type  B;  but  no 
sharp  line  could  be  drawn  between  the  blends  and  the  mosaics,  as  the  two 
graded  into  each  other  and  into  the  two  parental  types.  These  intermediates 
(class  C)  were  about  as  numerous  as  the  animals  of  type  D,  but  only  about 
half  as  numerous  as  those  of  type  B.  Thus,  in  a  particular  experiment  there 
were  produced  twenty-nine  B,  twelve  C  and  ten  D  individuals,  together  with 
eleven  of  unknown  character,  because  they  died  or  were  disposed  of  before 
their  hair  was  fully  grown.  The  grandparents  of  this  lot  were  the  long- 
haired male,  2002  (Castle,  105,  PI.  i,  Fig.  i),  and  several  different  short- 
haired  females ;  the  parents  were  all  of  class  B.  The  Mendelian  expectation 


10  HEREDITY   OF  HAIR-LENGTH   IN   GUINEA-PIGS. 

from  an  experiment  like  the  foregoing  is  that  there  will  be  produced  three 
times  as  many  animals  of  type  B  as  of  type  D,  or  in  a  total  of  fifty-one 
young  (the  number  whose  character  was  definitely  ascertained)  thirty-eight 
B  to  thirteen  D.  Comparing  with  these  numbers  the  observed  ones  (twenty- 
nine  B  and  ten  D),  we  notice  that  there  are  fewer  individuals  than  expected 
in  each  of  the  classes  B  and  D,  from  which  we  may  conclude  that  each  has 
contributed  to  the  formation  of  the  intermediate  class  C.  The  hypothesis 
on  which  the  Mendelian  expectation  rests  is  this,  that  each  cross-bred  animal 
will  form  in  equal  numbers  gametes  bearing  the  pure  B  character  and  the 
pure  !D  character  respectively.  Evidently  from  this  experiment  it  follows 
either  that  some  of  the  expected  B  and  D  gametes  are  not  pure,  or  else  that 
in  this  generation  the  result  of  a  union  between  a  B  and  a  D  gamete  is  not 
the  same  as  in  the  original  cross.  In  either  case  we  are  forced  to  admit 
modification  of  gametes  from  their  original  pure  condition. 

In  two  other  families  of  guinea-pigs,  cross-breds  mated  inter  se  pro- 
duced a  result  similar  to  that  already  described.  The  young  in  these  families 
were  classified  as  nine  B,  five  C,  two  D  and  four  undetermined.  These  cases 
differ  from  the  foregoing  in  that,  in  producing  the  cross-breds  used,  two 
crosses  with  the  B  race  were  made  to  one  with  the  D  race,  whereas  in  pro- 
ducing the  first  mentioned  lot  of  cross-breds  the  B  and  D  stocks  were 
equally  represented  in  the  ancestry.  It  is  perhaps  significant  that  the  D 
young  are  relatively  fewer  in  these  families,  though  the  total  is  too  small 
to  allow  one  to  attach  much  importance  to  the  proportions  observed  among 
the  young. 

(c)   MATING  B(D)  XD. 

On  the  Mendelian  hypothesis  of  pure  gametes,  this  cross  should  yield 
classes  B  and  D  in  equal  numbers.  In  matings  in  which  three  different  D 
males  were  employed,  it  has  produced  fourteen  B,  seventeen  C  and  nineteen 
D  offspring,  together  with  five  of  undetermined  character.  Since  D  parents 
are  known  to  form  only  D  gametes,  it  seems  clear  from  this  experiment  that 
the  hybrid,  or  B(D)  parent,  formed  gametes  a  considerable  number  of 
which  bore  the  intermediate  or  C  character.  If  a  single  cross  of  D  with  B 
has  such  an  effect  in  modifying  gametes,  a  repetition  of  the  cross  should 
have  a  still  more  marked  influence,  producing  a  still  larger  proportion  of 
intermediate  or  C  gametes.  The  mating  next  to  be  described  bears  on 
this  question. 

(d)   MATING  B(D)    [FROM  Two  SUCCESSIVE  CROSSES  WITH  B]XD. 
Three  different  D  males  were  used  in  making  this  cross.     They  pro- 
duced thirty-two  B,  thirty-seven  C,  and  twenty-nine  D  offspring,  together 
with  seven  of  undetermined  character.     Here  the  C  class  is  actually  larger 


HEREDITY   OF  HAIR-LENGTH   IN   GUINEA-PIGS.  II 

than  either  of  the  others,  though  the  increase  over  the  last  mating  is  not  a 
very  striking  one,  and  several  individuals  were  just  on  the  line  between 
C  and  D,  so  that  little  stress  can  be  laid  upon  the  classification  made. 

(e)  MATING  B(D)  [FROM  Two  SUCCESSIVE  CROSSES  WITH  D]  XD. 

If  two  crosses  introduce  greater  contamination  of  the  gametes  than 
one,  then  the  C  and  D  classes  resulting  from  this  cross  should  be  high  at 
the  expense  of  the  B  class.  The  observed  result  accords  with  this  inter- 
pretation, though  the  number  of  young  produced  is  not  large.  There  were 
one  B,  three  C  and  five  D  offspring. 

The  question  now  arises,  What  is  the  nature  of  the  C  individuals  ?  Are 
they  the  result  of  partial  reversal  of  dominance,  so  that  when  B  meets  D  in 
fertilization  there  is  produced  an  intermediate  condition,  or  have  B  and  D 
actually  fused  to  form  something  different  from  either  ?  Matings  of  C  indi- 
viduals among  themselves  or  with  B  and  with  D  respectively  should  throw 
light  on  this  question.  If  C  individuals  result  merely  from  partial  arrest 
of  the  dominance  of  B,  we  shall  expect  C  to  split  at  gamete  formation  into 
B  and  D.  If  it  does  not  do  this  we  may  conclude  that  it  represents  a  firm 
union  of  B  and  D  to  form  a  new  character,  C. 

(/)  MATING  C(£>?)  XD. 

This  mating  produced  a  mixture  of  C's  and  D's,  in  all  eight  C,  nine  D 
and  five  individuals  of  undetermined  character.  Apparently  the  C's  used  in 
this  experiment  were  heterozygous,  producing  some  C  and  some  D  gametes. 
The  former,  combined  with  D  gametes,  produced  C  individuals ;  the  latter 
similarly  united  produced  D  individuals.  The  fact  that  no  B  offspring  were 
produced  indicates  that  the  C  parents  did  not  form  B  gametes.  We  con- 
clude that  the  C  gamete  is  probably  a  new  creation  due  to  a  partial  and 
permanent  blend  of  B  with  D. 

(g)   MATING  C(D)    [PRODUCED  BY  Two  SUCCESSIVE  CROSSES  WITH  D]  XD. 

This  mating  should  yield  a  larger  proportion  of  D  offspring  than  the 
last,  if  cross-breeding  introduces  contamination  of  gametes.  The  observed 
result  was  nine  C,  eighteen  D  and  six  young  of  undetermined  character, 
which  result  supports  the  hypothesis  stated.  It  should  be  said  that  in  this 
mating,  as  in  those  previously  described,  the  classification  of  the  offspring 
was  wholly  unbiased,  as  it  was  in  each  case  made  before  the  animal's  ped- 
igree was  looked  up  to  see  in  what  group  of  offspring  the  individual  in 
question  should  be  placed.  It  must  be  borne  in  mind,  however,  that  the  C 
group  it  not  a  natural  one  to  be  considered  a  unit-character  by  itself.  It 


12  HEREDITY   OF   HAIR-LENGTH   IN   GUINEA-PIGS. 

is  simply  a  poorly  developed  D,  C  and  D  being  grades,  arbitrarily  fixed,  of 
the  continuous  growth  type  of  hair.  Or,  if  we  think  of  C  as  a  synthesis  of 
B  with  D,  then  this  C  partakes  more  largely  of  the  character  of  D  the 
more  often  the  parental  B  has  been  crossed  with  D  in  its  production.  From 
the  fact  that  the  C  parents  give  off  gametes  partaking  in  different  degrees 
of  the  D  character,  it  seems  probable  that  the  synthesis  of  B  and  D  to  form  C 
has  been  as  yet  imperfect,  as  of  two  ingredients  incompletely  mixed  together, 
so  that  different  samples  contain  different  proportions  of  the  D  character. 
This  view  is  supported  by  the  result  obtained  by  mating  C  with  C,  but  as 
the  parents  used  in  this  experiment  were  of  different  origin  from  those  men- 
tioned in  the  foregoing  pages,  it  may  be  well  first  to  describe  the  source 
from  which  they  came. 

4.    ORIGIN   OF   RACE   C',  AND   RESULTS   OF   CROSSES   IN   WHICH   IT   WAS 

EMPLOYED. 

A  family  of  short-haired  guinea-pigs  (class  B),  when  inbred,  produced 
a  few  individuals  with  hair  about  twice  as  long  as  that  of  their  parents. 
The  long  hairs  were  not  very  numerous.  It  seems  probable,  in  the  light  of 
subsequent  studies,  that  only  a  few  of  them  were  continuous  in  growth, 
the  others  being  determinate,  but  attaining  a  greater  length  than  do  ordi- 
nary hairs  before  growth  ceased.  When  two  of  these  long-haired  animals 
were  mated  together,  all  the  offspring  were  long-haired,  though  some  of 
them  had  more  numerous  long  hairs  than  others,  or  hairs  of  a  greater  maxi- 
mum length.  By  selecting  the  best  long-haired  individuals  for  two  genera- 
tions a  race  of  imperfectly  long-haired  individuals  was  produced,  of  about 
the  same  degree  of  excellence  as  the  group  C  already  described,  which  was 
produced  by  cross-breeding  between  B  and  D.  To  distinguish  it  from  the 
latter,  we  may  call  this  race  C '. 

The  C'  race,  bred  by  itself,  produced  no  B  individuals,  though  the 
maximal  hair  lengths  obtained  varied  considerably.  This  result  shows 
clearly  that  no  B  gametes  were  produced  by  the  C'  race,  though  it  had  just 
arisen  from  the  B  race,  in  which  very  likely  it  had  previously  been  present 
as  a  recessive  character. 

A  mating  between  C'  and  D  gave  a  mixture  of  C  and  of  D  individuals, 
the  two  classes  being  about  equally  numerous  (seven  C  to  five  D  in  one 
set  of  experiments).  Certain  individuals,  however,  were  just  on  the  line 
between  the  two  classes,  so  that  no  great  importance  can  be  attached  to 
the  proportions  observed.  But  the  result  does  show,  what  breeding  C  or  C 
individuals  inter  se  had  shown,  a  considerable  degree  of  variability  among 
the  C  and  C  gametes,  this  variability  being  a  sufficient  basis  for  selection 
for  increased  hair-length.  While,  accordingly,  the  variation  made  its  appear- 
ance as  a  discontinuous  one,  it  showed  itself  amenable  to  selection. 


HEREDITY   OF   HAIR-LENGTH    IN   GUINEA-PIGS.  13 

5.     GAMETES   OF   CROSS-BREDS   OFTEN    IMPURE. 

From  the  foregoing  observations  it  is  clear  that,  while  the  long-haired 
and  short-haired  conditions  are  sharply  alternative  to  each  other  in  heredity, 
the  gametes  formed  by  cross-breds  are  not  in  all  cases  pure.  Frequently 
they  consist  of  a  blend  or  a  mixture  of  the  two  alternative  conditions,  con- 
stituting in  effect  a  new  condition  intermediate  between  the  other  two.  A 
study  of  other  characters  alternative  in  heredity  yields  results  somewhat 
similar. 

Albinism  is,  in  heredity,  the  most  sharply  alternative  of  characters,  yet 
cross-breeding  between  albino  and  pigmented  guinea-pigs  may  modify  the 
character  both  of  the  albino  race  and  of  the  pigmented  one.  This  modifi- 
cation may  take  on  a  variety  of  forms,  as  has  elsewhere  been  pointed  out 
(Castle,  105).  It  may  result  in  the  production  of  mosaics  (pigmented 
animals  spotted  with  white),  or  of  albinos  with  a  modified  peripheral 
pigmentation,  or  of  albinos  visibly  like  their  ancestors  but  transmitting  a 
different  set  of  latent  characters.  Again,  the  rough  or  resetted  coat  of  certain 
races  of  guinea-pigs  is  sharply  alternative  to  smooth  coat,  yet  cross-breeding 
of  rough  with  smooth  races  may  induce  curious  modifications  of  the  rough 
character  or  produce  smooth  individuals  bearing  the  merest  trace  of  the 
rough  character. 

All  these  facts  are  in  harmony  with  the  hypothesis,  for  which  there  is 
strong  evidence  on  the  cytological  side,  that  each  separately  heritable  char- 
acter is  represented  by  a  different  structural  element  in  the  germ  (egg  or 
spermatozoon).  In  fertilization  the  paternal  and  maternal  representatives 
of  a  character  become  more  or  less  closely  united,  this  union  persisting 
through  all  subsequent  cell-generations  until  the  new  individual  forms  its 
sexual  elements.  At  that  time  the  paternal  and  maternal  representatives  of 
a  character  separate  from  each  other  and  pass  into  different  cells. 

But  the  paternal  and  maternal  representatives  of  a  character  may  in 
the  meantime  have  exercised  on  each  other  a  considerable  influence.  In 
the  case  of  some  characters,  as  ear-length  in  rabbits  (Castle,  :o5a),  they  com- 
pletely blend  and  intermingle,  so  that  a  new  character  is  produced  strictly 
intermediate  between  the  conditions  found  in  the  respective  parents. 

In  other  cases  the  modification  may  be  slight,  as  if  the  paternal  and 
maternal  representatives  of  a  character  had  been  scarcely  more  than  approx- 
imated. Sometimes  in  cases  of  alternative  inheritance  no  influence  of  the 
cross  is  observable  in  certain  of  the  "  extracted  "  individuals,  but  if  any  con- 
siderable number  of  individuals  is  examined,  others  will  be  found  in  which 
the  cross-breeding  manifests  its  influence.  From  this  we  conclude  that 
gametic  purity  is  not  absolute,  even  in  sharply  alternative  inheritance. 


HEREDITY  OF   HAIR-LENGTI 


6.    BIBLIOGRAPHY. 

CASTLE,  W.  E. 

:O3-    The  heredity  of  "  Angora "  coat  in  mammals.    Science,  n.  s.,  vol.  18,  no. 

467,  pp.  760-761.    December  II,  1903. 
CASTLE,  W.  E. 

105.    Heredity  of  coat  characters  in  guinea-pigs  and  rabbits.    Carnegie  Institu- 
tion publication  no.  23,  78  pp.,  6  plates  and  8  text-figures.    February,  1905. 
CASTLE,  W.  E. 

105*.    Recent   discoveries   in   heredity   and   their   bearing   on   animal   breeding. 

Pop.  Sci.  Monthly,  July,  1905;  pp.  193-208,  14  figs. 
DAVENPORT,  C.  B. 

:04-    Wonder  horses  and  Mendelism.    Science,  n.  s.,  vol.  19,  no.  473,  pp.  i5I~I53- 
January  22,  1904. 


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